Recently, there has been an increasing interest in energy storage technology day by day. As the application field of energy storage technology has been extended to mobile phones, camcorders, lap-top computers, and even electric cars, many efforts have been devoted to studying and developing electrochemical devices. In this aspect, electrochemical devices are attracting more attention, and especially, development of rechargeable secondary batteries is the focus of attention.
Electrochemical devices have been long studied, and those with greatly improved performance of electrode active materials, especially output, have been developed. In currently available secondary batteries, lithium secondary batteries developed in early 1990's have higher operating voltage and higher energy density a traditional batteries such as Ni-MH batteries, and by virtue of these advantages, lithium secondary batteries are gaining much attention. However, lithium secondary batteries have risks of heat generation and eventual explosion due to the environment in which the batteries are used.
Electrochemical devices are produced by many manufacturers, and each shows different safety characteristics. Assessment and management of the safety of electrochemical batteries is very grave. The most important consideration is that electrochemical devices should not cause injury to users in the event of malfunction, and for this purpose, Safety Regulations strictly prohibit fire and flame in electrochemical devices. In the safety characteristics of electrochemical devices, overheating and eventual thermal runaway in electrochemical devices or piercing of separators poses a high risk of explosion. Particularly, polyolefin-based porous substrates commonly used for separators of electrochemical devices show extremely severe thermal shrinking behaviors at the temperature of 100° C. or above due to their properties of materials and manufacturing processes including stretching, causing a short circuit between positive and negative electrodes.
To solve the safety problem of electrochemical devices, Korean Patent Publication Nos. 10-2006-72065 and 10-2007-231 proposed an organic/inorganic composite separator having a porous active layer, in which the porous active layer is formed by coating a mixture of inorganic particles and a binder polymer on at least one surface of a porous substrate having a plurality of pores. In the organic/inorganic composite separator, the inorganic particles in the porous active layer coated on the porous substrate serve as a sort of spacer to mechanically maintain the shape of the porous active layer, thereby suppressing the thermal contraction of the porous substrate when the electrochemical device is overheated. Furthermore, interstitial volume between the inorganic particles forms micropores.
As described above, for the porous active layer coated on the organic/inorganic composite separator to suppress the thermal contraction of the porous substrate, the inorganic particles need to be present in a predetermined amount or more. However, as the content of the inorganic particles becomes higher, the content of the binder polymer becomes relatively lower, and thus the inorganic particles of the porous active layer may get unstuck due to stress occurring in the assembly process of the electrochemical device including winding. The unstuck inorganic particles act as local defects of the electrochemical device and have a bad influence on the safety of the electrochemical device.